There is a radar apparatus which receives reflected waves obtained through reflection of transmitted waves from an object. The radar apparatus, which has received the reflected waves, generates a beat signal that represents a difference between a transmitted signal corresponding to the transmitted waves and a received signal corresponding to the received waves. A signal processing apparatus of the radar apparatus performs fast Fourier transform of the beat signal to derive transformed data. Further, the signal processing apparatus derives a signal having a level that exceeds a predetermined threshold value (hereinafter referred to as a “peak signal”) in the transformed data in a first period where the frequency of the transmitted signal ascends and in a second period where the frequency descends, and calculates a position of a detection point pertaining to the object through calculation of the derived peak signal.
On the other hand, in the case where the radar apparatus is provided with a plurality of array antennas, one of algorithms of DBF (Digital Beam Forming), PRISM (Propagator method based on an Improved Spatial-smoothing Matrix), MUSIC (Multiple Signal Classification), and ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) is used to calculate an angle of the detection point. For example, in the case of using the ESPRIT among the above-described algorithms, spatial smoothing preprocessing may be used to derive a correlation matrix that is used to calculate the angle of a detection point.
The spatial smoothing preprocessing is a process of deriving ideal signal components that correspond to desire waves by reducing mutual interference components of received signals through generation of a plurality of correlation matrices by means of a sub-array that is a combination of a predetermined number of antennas among a plurality of array antennas and derivation of an average of the correlation matrices through addition of the correlation matrices. That is, by adding the correlation matrices based on transformed data acquired through performing of the fast Fourier transform of the signals received from the reception antennas, an average value of the correlation matrices is derived, and thus signals having a reduced influence of the mutual interference components are derived.
Further, in the case where the first period where the frequency of the transmitted signal ascends and the second period where the frequency descends constitute one period of the transmitted signal, the number of correlation matrices that are generated in proportion to the increase in the number of continuous periods of the transmitted signal is increased, and the number of correlation matrices that are used in the spatial smoothing preprocessing is also increased. Further, as the number of correlation matrices used in the spatial smoothing processing becomes larger, the reduction rate of the mutual interference components of the received signal is also increased. Accordingly, it is preferable to generate the correlation matrices through increasing of the number of continuous periods as large as possible. That is, by increasing the continuous output time of the transmitted waves that correspond to the transmitted signal, the correlation matrices are generated in each period of the transmitted signal, and the average value of the correlation matrices is derived through addition of all the correlation matrices. Accordingly, the mutual interference components of the received signal is greatly reduced, and thus it becomes possible to derive respective angles of a plurality of detection points pertaining to the object can be exactly derived with high accuracy. On the other hand, as a material that describes the technology according to the present invention, there is Patent Document 1.
Patent Document 1: Japanese Patent Application Publication No. 2010-025928
However, if the time for which a radar apparatus of 1 continuously transmits the transmitted waves is increased, the possibility that another radar apparatus receives at least one of the transmitted waves and the reflected waves from the radar apparatus of 1 would become higher. In this case, another radar apparatus may detect incorrect detection points.
Further, as the time for which the radar apparatus continuously transmits the transmitted waves is increased, the amount of heat generated inside the radar apparatus is increased. Due to this, the load on components constituting the radar apparatus is increased. Accordingly, it is required to shorten the time when the transmitted waves are continuously output as short as possible, that is, to reduce the number of periods where the transmitted signal is continuous. However, if the time for which the transmitted waves are continuously output is shortened, the number of correlation matrices used in the spatial smoothing preprocessing is reduced, and thus the angles of the detection points may not be accurately derived.